How do you unselect vertices without a middle mouse button?
In Blender you really have to have a middle-mouse-button, but if you don't own a 3 button mouse, you can emulate one in the preferences.


Open User Preferences Control+Alt+u.
Switch the the Input tab.
Enable Emulate 3 Button Mouse.


You can now use Alt+Left-Mouse to perform tasks normally done with the middle mouse button.